Suppose an illness that can affect 1% of the people. Also assume that there is a test for that illness, that gives the correct result 99% of the times.

If you take that test, and receive a POSITIVE result, should you worry much?

If you take it again, and once more get a POSITIVE, should you worry then?

How many consecutive POSITIVEs would you have to get in order to be sure that the chances of a wrong diagnostic are 1 in a million?

(In reply to Combined probabilities by François)

"Actually, after step number 4, the probability is useless in this example because the population of the world is not big enough for these results to mean anything in statistics."

In other words, for the example of 10,000 that Charlie used to show the first case; trying to do a similar example to show the results of five or more iterations would require an example size larger than Earth's population to find just one person who tested positived inaccurately.

Posted by DJ on 2005-01-07 12:51:58